In recent years, the performance of electronic components in the electronics field has been greatly enhanced, contributing to high-speed information processing, compact design, and multifunctional design of information processing apparatuses such as mainframes, mobile communicating terminals, and personal computers. A multichip module (MCM), having multiple semiconductor devices such as VLSIs and ULSIs on its ceramic substrate, is one of these electronic components. In such a module, a ceramic multilayer substrate having wiring conductors arranged three-dimensionally is often used in order to increase the packaging density of LSIs and achieve good electric connections between LSIs.
A ceramic multilayer substrate is produced by firing a green multilayer collective substrate formed of a plurality of laminated ceramic green sheets. Unfortunately, green multilayer collective substrates, when fired by a conventional technique, shrink in the directions along the main surfaces and across the thickness, resulting in dimensional errors of about 0.4% to 0.6% particularly in the directions along the main surfaces. This may cause outer conductors to suffer from a degraded positional accuracy and inner conductors to be deformed, distorted, or broken.
In view of the problems described above, Japanese Unexamined Patent Application Publication No. 4-243978 discloses a method for manufacturing ceramic multilayer substrates, which is described below.
First, glass ceramic powder that is sinterable at temperatures of 1000° C. or less and alumina powder that is not sintered at the sintering temperature of this glass ceramic powder are prepared. Then, ceramic green sheets containing the glass ceramic powder are laminated on one another, and the resultant green multilayer substrate is then interposed between shrinkage-restraining layers containing the alumina powder. Thus, a green composite laminate is produced.
The composite laminate is then fired under the sintering conditions used for the glass ceramic powder. At this time, the alumina powder contained in the shrinkage-restraining layers is not substantially sintered, and therefore shrinkage does not substantially occur in the shrinkage-restraining layers. The above-mentioned effect causes the shrinkage-restraining layers to restrain the green multilayer substrate, so that the multilayer substrate shrinks across the thickness only; shrinkage in the directions along the main surfaces is suppressed. Removing the shrinkage-restraining layers thereafter by appropriate means allows the ceramic multilayer substrate to be prepared.
The ceramic multilayer substrate obtained by the non-shrinking process described above is highly reliable because it has high dimensional accuracy in the directions along the main surfaces, i.e., the lengthwise direction (X direction) and widthwise direction (Y direction) of the ceramic green sheets, and suffers from less camber and torsion.
In order to efficiently produce such a ceramic multilayer substrate as described above, a multilayer collective substrate including a plurality of ceramic multilayer substrates is first produced and is then split along predetermined split lines into a plurality of ceramic multilayer substrates.
In splitting the multilayer collective substrate as described above, grooves may be formed along respective predetermined split lines on a main surface of the multilayer collective substrate to facilitate the splitting of the multilayer collective substrate.
These grooves are normally formed on a main surface of the green multilayer collective substrate using a cutter or a die. When the ceramic multilayer substrate is produced using shrinkage-restraining layers, as described above, grooves are formed as shown in FIG. 3, which is disclosed in Japanese Unexamined Patent Application Publication No. 7-99263.
FIG. 3 is a sectional view showing a part of a green composite laminate 101 which includes a green multilayer collective substrate 102, a first shrinkage-restraining layer 103, and a second shrinkage-restraining layer 104, the green multilayer collective substrate 102 being interposed between the layers 103 and 104. In FIG. 3, wiring conductors provided on the multilayer collective substrate 102 are not shown and the dimension across the thickness is enlarged.
In FIG. 3, the green multilayer collective substrate 102 includes a plurality of ceramic green sheets 107 containing glass ceramic powder. The first shrinkage-restraining layer 103 and the second shrinkage-restraining layer 104 are formed of a predetermined number of laminated green sheets 108 containing sintering-resistant powder such as alumina powder which is not sintered at the sintering temperature used for the above-mentioned glass ceramic powder.
This green composite laminate 101 is manufactured as follows.
First, the ceramic green sheets containing glass ceramic powder are laminated and compressed in the lamination direction, and thereby the green multilayer collective substrate 102 containing a plurality of the ceramic green sheets 107 is produced. Next, grooves 106 are formed on one main surface of the green multilayer collective substrate 102.
Then, the green sheets 108 containing alumina powder are laminated so as to interpose the green multilayer collective substrate 102, thus the first shrinkage-restraining layer 103 and the second shrinkage-restraining layer 104 are formed. As a result, the green composite laminate 101 is obtained. Subsequently, the green composite laminate 101 is compressed again in the lamination direction, and is then fired at the sintering temperature used for the glass ceramic powder contained in the ceramic green sheets 107.
Then, the first shrinkage-restraining layer 103 and the second shrinkage-restraining layer 104, which are not substantially sintered in the firing step, are removed to produce the sintered multilayer collective substrate 102 with the grooves 106. The sintered multilayer collective substrate 102 is split along the grooves 106 to prepare individual ceramic multilayer substrates.
Before the grooves 106 are formed, as described above, on the green multilayer collective substrate 102, a plurality of the laminated ceramic green sheets need to be pre-compressed sufficiently in order to prevent the individual ceramic green sheets 107 from shifting at the time the grooves are formed.
However, when the green composite laminate 101 is compressed again, it is difficult to achieve a sufficiently tight bond between the compressed green multilayer collective substrate 102 and the uncompressed first shrinkage-restraining layer 103 and the second shrinkage-restraining layer 104. As a result, the force by which the first shrinkage-restraining layer 103 and the second shrinkage-restraining layer 104 restrain the ceramic green sheets 107 becomes small, possibly failing to satisfactorily prevent shrinkage of the ceramic green sheets 107 during firing, i.e., shrinkage of the green multilayer collective substrate 102 in the surface directions.
Another disadvantage is a low productivity arising from an unusual step of forming the grooves 106 intervening the step of laminating the ceramic green sheets 107 constituting the multilayer collective substrate and the green sheets 108 constituting the shrinkage-restraining layers.
In view of the problems described above, an object of the present invention is to provide a method for manufacturing ceramic multilayer substrates with a high dimensional accuracy and a high reliability in a highly productive manner and to provide a green composite laminate obtained at a step in this method.